Wii is number one, but every week 200k more HD Consoles are in households. Madden, Medal of Honor, and all the other fall backs of the market sell better on HD consoles.
I love my Wii, but you are in dreamland if you think that the core games are as a attractive to develop for Wii as far as third party profit is concerned you're crazy.
I think it's because hardcore gamers that own Wii's also own gaming PC's or HD Consoles. But I also think the variety of different experiences influences buyers. When I was at the store two weeks ago I had to decide between Boom Blox, Wii Fit (preorder), Okami, etc.
Where for PC I am choosing between RTS 1 and RTS2 or FPS1 or FPS2 and I pick the one I have played at a friends house or which got the higher review score.